I'm doing a remodel and there is a existing 6-2 wire that is not used. Can I use that as the feed for a 20amp branch circuit?

12-2 should be plenty. I'd replace the big wire if its easily accessible. Be careful making a connection to this big wire to your device and you'll be okay if its copper.

Using a #6 AWG conductor for a 20 Amp circuit can be done, but it is folly to do so. #6 AWG is listed for a 60 amp circuit #12/2 w/ground is listed for a 20 Amp circuit.

#6 AWG conductors are extremely difficult to work with because of the stiffness of the conductor. Also, you would have to splice #12 AWG conductors to it on both ends of the cable to be able to attach it to anything. Unless of course you are trying to drive the 20 amps 200 feet.

Replace the cable!

Using a #6 AWG conductor for a 20 Amp circuit can be done, but it is folly to do so. #6 AWG is listed for a 60 amp circuit #12/2 w/ground is listed for a 20 Amp circuit.


#6 AWG conductors are extremely difficult to work with because of the stiffness of the conductor. Also, you would have to splice #12 AWG conductors to it on both ends of the cable to be able to attach it to anything. Unless of course you are trying to drive the 20 amps 200 feet.

Replace the cable!
Replacing the #6 AWG is not a option.

Replacing the #6 AWG wire is not a option.

... then shouldn't your question be, "how do I use it...."
Splice on pieces of #12 on each end and make sure the circuit breaker is 20 amperes or less... It's somewhat unusual but it is legal.